Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-06-13
2003-07-15
Mack, Ricky (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S246000
Reexamination Certificate
active
06594062
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to photoluminescent polarizers and in particular to photoluminescent polarizers which are characterized in a low degree of polarization in their absorption and a high degree of polarization in their emission. The invention also relates to methods to produce the latter. Also, the invention relates to the application of these photoluminescent polarizers in display devices.
BACKGROUND OF THE INVENTION
Linear sheet polarizers, which convert unpolarized into linearly polarized light are well known in the art and are of major importance in a large variety of applications (L. K. M. Chan in “The Encyclopedia of Advanced Materials”, Vol. 2, D. Bloor, T. J. Brook, M. C. Flemings, S. Mahajan, eds., pp. 1294-1304 (1994), Elsevier Science Ltd., Oxford; D. S. Kliger et al. “Polarized Light in Optics and Spectroscopy” (1990), Academic Press, San Diego; T. J. Nelson et al. “Electronic Information Display Technologies” (1997), World Scientific Publishing, Singapore). However, the presently employed polarizers suffer from severe limitations some which are summarized below.
The vast majority of linear sheet polarizers presently used, are dichroic polarizers which are based on an invention by Land et al. (E. H. Land, J. Opt. Soc. Am., Vol. 4, pp. 957 (1951)). As well established in the prior art, dichroic polarizers are produced from oriented, synthetic polymers which contain oriented dichroic species. Dichroic polarizers operate by the absorption of one polarization direction of incident light, thus, a dichroic polarizer which generates perfectly, linearly polarized light absorbs 50% or more of unpolarized, incident light (D. S. Kliger et al. “Polarized Light in Optics and Spectroscopy” (1990), Academic Press, San Diego). Consequently, dichroic polarizers convert at least 50% of the incident optical energy into heat which severely limits the efficiency of these polarizers and causes problems due to the excessive heating in combination with high-intensity light sources.
As an alternative to dichroic polarizers, polarizers have been proposed that are based on selective reflection or scattering of one polarization and allow recycling of the reflected or scattered light (European Patent EP 0 606 940 A2; World Patent WO 9735219 A1; U.S. Pat. Nos. 5,325,218; 5,422,756; 5,528,720; 5,559,634; M. Schadt et al., Jap. J. Appl. Phys., Vol. 29, pp. 1974-1984 (1990); D. J. Broer et al., Nature, Vol. 378, pp. 467-469 (1995); D. Coates et al., SID 96 Applications Digest, pp. 67-70 (1996)) or scattering (Y. Dirix, “Polarizers based on anisotropic absorbance or scattering of light”, Ph. D. thesis, Technische Universiteit Eindhoven, Eindhoven. The Netherlands (1997)). However, these polarizers also suffer a number of severe drawbacks. All above referred reflecting or scattering polarizers, due to their working principle, require additional light-recycling systems and other additional elements which render them rather uneconomical. Some of these polarizers initially produce circularly rather than linearly polarized light (D. J. Broer et al., Nature, Vol. 378, pp. 467-469 (1995)) and require expensive quarter-wave converters to produce linearly polarized light, or are manufactured by processes with intrinsic limitations for the production of large area, flexible polarizing films.
As is well known in the art, the production of linearly polarized, chromatic (colored) light, which is essential for many technical applications, including liquid-crystal displays, presents another obstacle. Polarized, colored light is usually obtained by the use of multiple elements: a polarizer and one or multiple color filters (L. K M. Chan in “The Encyclopaedia of Advanced Materials”, Vol. 2, D. Bloor, T. J. Brook, M. C. Flemings, S. Mahajan, eds., pp. 1294-1304 (1994), Elsevier Science Ltd., Oxford). The vast majority of color filters presently used are absorbing color filters which convert a major fraction, i.e. usually 80%, of incident light into thermal energy (T. J. Nelson et al. “Electronic Information Display Technologies” p. 244 (1997), World Scientific Publishing, Singapore) and, thus, also create severe limitations with respect to energy efficiency, brightness and accumulation of thermal energy. As an alternative to absorbing color filters, the use of photoluminescent (PL), for example fluorescent or phosphorescent matter as “active” color filters has also been described (German patent No. DE 2640909 C2; French application FR 2 600 451-A1: U.S. Pat. Nos. 3,844,637; 4,113,360; 4,336,980; 4,394,068; 4,470,666; 4,678,285; 5,018,837; 5,608.554; G. Baur et al., Appl. Phys. Lett., Vol. 31, pp. 4-6 (1977); M. Bechtler et al., Electronics, December 8, pp. 113-116 (1977); W. Greubel et. al., Elektronik, pp. 55-56 (1977); H. J. Coles, Liq. Cryst., Vol. 14, pp. 1039-1045 (1993); W. A. Crossland et al., Proc. SID Symp. Digest of Technical Papers, Vol. 27, pp. 837-840 (1997)). However, the proposed structures suffer from a number of drawbacks that are related to the limited stability and efficiency of the fluorescent dyes the difficulty to produce structured materials, depolarization effects, or the required thickness and (large) area of the luminescent layer.
Recently, some PL materials have been demonstrated to combine the functions of a linear polarizer and a color filter and to yield linearly polarized, chromatic light in one single element (Ch. Weder et al., Adv. Mat., Vol. 9, pp. 1035-1039 (1997)). When processed into appropriate forms, these PL materials can be used as PL polarizers which lead to a substantial increase in device brightness and efficiency when used instead of a dichroic polarizer and an absorbing color filter, for example in liquid-crystal display devices. In addition, PL polarizers offer a significant simplification in device design, because they combine the functions of two elements. The prior art PL polarizers comprise uniaxially oriented, formanisotropic, PL substances, which after photoexcitation emit linearly polarized light. This effect is well known in the art; it was demonstrated in inorganic crystals more than a century ago (E. Lommel, Ann. d. Physik und Chemie. Vol. 8. pp. 634-640 (1879)) and in oriented blends of ductile polymers and low-molecular weight PL materials as early as the 1930's (A. Jablonski, Acta Phys. Polon., Vol. A 14, pp. 421-434 (1934)). Since, the effect has been shown in a variety of systems (J. Michl et al. “Spectroscopy with polarized light” (1986), VCH Publishers, New York) including, for example, oriented blends of ductile polymers and oligomeric PL materials (M. Hennecke et al., Macromolecules, Vol. 26, pp. 3411-3418 (1993)), uniaxially oriented PL polymers (P. Dyreklev et al., Adv. Mat., Vol. 7, pp. 43-45 (1995)) or blends thereof and a ductile polymer (U.S. Pat. No. 5,204,038; T. W. Hagler et al., Polymer Comm., Vol. 32, pp. 339-342 (1991); T. W. Hagler et al. Phys. Rev., Vol. B 44, pp. 8652-8666 (1991); Ch. Weder et al., Adv. Mat., Vol. 9, pp. 1035-1039 (1997)), liquid crystal systems (N. S. Sariciftci et al., Adv. Mater., Vol. 8, p. 651 (1996); G. Lüssem et al., Adv. Mater., Vol. 7, p. 923 (1995)) or oriented PL materials grown onto orienting substrates (K. Pichler et al., Synth. Met., Vol. 55-57, p. 454 (1993); N. Tanigaki et al., Mol. Cryst. Liq. Cryst., Vol. 267, p. 335 (1995); G. Lüssem et al., Liq. Cryst., Vol. 21, p. 903 (1996); R. Gill et al., Adv. Mater. Vol. 9, pp. 331-334 (1997)). The efficiency of PL polarizers is limited by the quantum yield of the PL material which, in principle, can approach unity (B. M. Krasovitskii et al. “Organic Luminescent Materials” (1988), VCH, Weinheim). Unfortunately, the uniaxial orientation of the formanisotropic, PL substances in the PL polarizers, which have been described in the prior art, not only gives rise to an anisotropic, that is, linearly polarized, emission, buts also to an anisotropic absorption. Consequently, only one polarization direction of unpolarized incident light is optimally absorbed and used for photoexcitation, while the other polarization direction is, at
Bastiaansen Cees
Montali Andrea
Smith Paul
Weder Christoph
Landqart
Mack Ricky
Morgan & Lewis & Bockius, LLP
LandOfFree
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